This invention relates to chemical sensing apparatus and methods, and more particularly to apparatus comprising a solid-electrolyte, solvent-free, moisture-resistant amperometric sensor. Certain air contaminants may be either directly hazardous--e.g., acutely toxic or carcinogenic compounds, such as carbon monoxide (CO), formaldehyde (H.sub.2 CO), hydrazine (N.sub.2 H.sub.4), methyl hydrazine (CH.sub.3 N.sub.2 H.sub.3) or dimethyl hydrazine [(CH.sub.3).sub.2 N.sub.2 H.sub.2 ]--or they may be indicative of a hazard--e.g., methane (CH.sub.4) in coal mines or ethanol (CH.sub.3 CH.sub.2 OH) in the driver's seat of a moving motor vehicle. Early detection of such compounds may permit or trigger timely actions (e.g., disabling an automobile ignition system) to prevent death or injury.
Although various detectors for such compounds exist, they are usually bulky and expensive. Commercially available electrochemical sensors are relatively small and cheap, but they usually require an aqueous or non-aqueous liquid electrolyte solution which necessitates proper containment and periodic replenishments (following evaporation or other losses). To extend the time intervals between replenishments, a liquid reservoir is usually provided that is much larger than what is needed for the sensing function, which in turn enlarges the size and weight of the sensor. The sensors are exposed to the air that is to be monitored or analyzed and interact therewith so as to produce a detectable signal or change in the presence of selected compounds. The electrochemical detectors may be of either of two different types, namely potentiometric or amperometric. The potentiometric sensors are usually less accurate than amperometric sensors and have several other disadvantages.
Amperometric sensors, which exhibit current flow that is proportional to the concentration of an analyte, usually contain a liquid electrolyte, either aqueous or non-aqueous. The liquid electrolyte may sometimes be gelled or confined within a water-permeated ion-exchange polymer. Such sensors, as well as their operating mechanisms and associated detection or measuring circuitry, are well known in the art. Most heretofore disclosed solid-state, solvent-free amperometric sensors require elevated temperatures for operation.
Ambient-temperature amperometric sensors comprising the solid-state ionic conductor silver rubidium iodide (Ag.sub.4 RbI.sub.5) have been disclosed in Japanese Patent No. 142266, issued in June 1987, for the detection of nitric oxide (NO), and by Buttner et al. (Proceed-ings of the 1987 Scientific Conference on Chemical Defense Research, 17-20 November 1987, U.S. Army Chemical Research, Development, and Engineering Center, Aberdeen Proving Ground, MD, CRDEC-SP-88013, 1988, Vol. 2, pp. 1011-1017) for the detection of NO, CO, N.sub.2 H.sub.4 and CH.sub.3 N.sub.2 H.sub.3. However, the lifetime of the Ag.sub.4 RbI.sub.5 -based sensors turned out to be rather brief (&lt;3 months) due to instability of Ag.sub.4 RbI.sub.5 to moisture in he air.
Other reported solid-electrolyte sensors operate at elevated temperatures (&gt;100.degree. C.) and/or in the potentiometric mode. An amperometric sensor for the room temperature detection of methane is disclosed by Zaromb et al. in U.S. Pat. No. 4,591,414, issued on May 27, 1986. In this sensor, the potential of the sensing electrode is set high enough to effect the electro-oxidation of methane even at room temperature. However, the liquid non-aqueous electrolyte that is required to withstand the highly oxidizing working electrode potential presents confinement and replenishment problems.
It is therefore an object of this invention to provide an all-solid-state (i.e., substantially liquid-free) and room- or ambient-temperature-operating amperometric sensor for chemical detection that is not adversely affected by humidity.
It is another object of the invention to provide a chemical sensing apparatus comprising a long-lasting humidity-resistant solid-electrolyte amperometric sensor that operates at ambient temperature.
It is yet another object of the invention to provide a chemical sensing apparatus comprising an array of differently selective, miniaturized, solid-electrolyte sensors for the selective detection of various hazardous or hazard-indicative air contaminants.
It is still another object of the invention to provide apparatus for the detection of hazardous or hazard-indicative compounds such as CO, NO, H.sub.2 CO, N.sub.2 H.sub.4, CH.sub.3 N.sub.2 H.sub.3, (CH.sub.3).sub.2 N.sub.2 H.sub.2, CH.sub.4, or CH.sub.3 CH.sub.2 OH comprising one or more miniaturized, solid-electrolyte ambient-temperature amperometric sensors.
These and other objects of this invention will become apparent from the following description and appended claims.